A single reflex digital camera is disclosed in Patent Document 1, in which an optical object image formed by an image-forming optical system is formed on a two-dimensional light-receiving sensor such as a CCD sensor or a CMOS sensor, and an image output of the object is obtained by photoelectric conversion of the optical image.
A beam splitter, which transmits light of a visible wavelength region without reducing the light amount and divides light of a wavelength close to the infrared region, is built in the single reflex digital camera. The light of the wavelength close to the infrared region divided by the beam splitter is used for focus detection, and the straight-advancing light is used for image-taking. Limiting the area of a beam splitting function surface in such a beam splitter within an area where the focus detection luminous flux passes can reduce the thickness of the beam splitter. Thereby, the beam splitter can be disposed in a small space between the image-forming optical system and a mirror that deflects the optical path toward a viewfinder optical system, without increasing the size of the camera. In addition, setting the spectral transmittance characteristic of the beam splitter to approximately 100% in the visible wavelength region can produce a bright and high-quality image without reducing the light amount of the visible wavelength region that is required for image-taking of objects.
A camera with a zoom lens is disclosed in Patent Document 2, in which focus detection is performed using a luminous flux divided in the middle of an optical path of the zoom lens.
Generally, in an optical structure in which an optical path of a luminous flux taken through an image-forming optical system is divided into a plurality of optical paths and they are led to a light-receiving element, the substantially same wavelength characteristics of the divided luminous fluxes often produce an advantage.
Since the light of the wavelength region close to the infrared region divided by the beam splitter is used for focus detection in the camera disclosed in Patent Document 1, aberration correction of the image-forming optical system in this wavelength region is required for performing the focus detection correctly. It is impossible to focus the camera exactly using the light of the wavelength close to the infrared region in a case where the aberration correction is not enough. On the other hand, the aberration correction in the wavelength region close to the infrared region in addition to the visible wavelength region requires use of a special glass or increase in the number of constituent lenses of the image-forming optical system, and thereby increasing in cost and size of the camera. This is not preferable. Especially, in a case where the image-forming optical system is interchangeable like a single reflex camera and a large-scale interchangeable lens system is provided for the camera, it is very difficult to realize the enough aberration correction in both the wavelength region close to the infrared region and the visible wavelength region because the entire interchangeable lens system must correspond to such a focus detection system.
In addition, the measurement of object luminance using the luminous flux divided by the beam splitter and the determination of image-taking exposure amount causes a phenomenon similar to the focus detection. In other words, an image-taking result with underexposure or overexposure will occur even if an image-taking is performed with the exposure amount determined based on the measurement of object luminance because the exact estimation of the light energy included in a wavelength region for the exposure from the light energy included in a wavelength region for the luminance measurement cannot be performed when the wavelength region for the exposure displaces from the wavelength region for the luminance measurement.
Such a problem can be prevented in principle if the wavelength characteristics of the divided luminous fluxes are substantially the same. In other words, it can be prevented if the spectral transmittance characteristic of the beam splitting function surface in the beam splitter becomes flat for a desired wavelength region.
[Patent Document 1]
Japanese Patent Laid-open Application No. 2003-140246
[Patent Document 2]
Japanese Patent Laid-open Application No. H63-195630
By the way, the area of the beam splitting function surface required for focus detection and object luminance measurement is generally set as a part of area through which a luminous flux for image-taking passes. However, actually, if the area of the beam splitting function surface is not set so as to cover the entire luminous flux for image-taking, uneven luminance will occur in an image taken by using the straight-advancing light, and thereby reducing the image quality. In addition, although uneven luminance will be hard to occur when the beam splitter is disposed in the vicinity of the pupil plane of the image-forming optical system, the contrast of the image will reduce because the amount of light passing through the center of the pupil reduces.
FIG. 30 is a pattern diagram of an optical apparatus, which shows an appearance of generation of the uneven luminance. In this figure, Reference numeral 901 denotes an image-forming optical system, 902 a light-receiving element, which has sensitivity to only the visible light, such as a photo film or a CCD sensor, 903 a beam splitter. The image taken by the light-receiving element 902 becomes a negative plate by developing if the optical apparatus is a film camera, or, is displayed on an electric viewfinder (EVF), recorded in a memory or printed out if the optical apparatus is a digital camera.
A dielectric multilayer film is formed on a beam splitting function surface 903a of the beam splitter 903. The 50% of the visible light component of the object light emerged from the image-forming optical system 901 is reflected by the beam splitting function surface 903a, and the remaining 50% of the visible light component is transmitted through the beam splitting function surface 903a. The light reflected by the beam splitting function surface 903a is totally reflected by a surface 903b of the beam splitter 903, and then passes through a surface 903c to emerge out of the beam splitter 903.
If the optical apparatus is an infrared camera, the light-receiving element 902 has sensitivity to only infrared light, and the beam splitting function surface 903a of the beam splitter 903 divides the infrared light.
In FIG. 30, since the amount of the visible light transmitted through the beam splitting function surface 903a reduces by half, if the object for image-taking is an even luminance surface, the image taken by the light-receiving element 902 will become like an image 910 in which bright areas 912 and 913 are formed above and below the dark center area 911. Such a phenomenon is not preferable because an unnatural luminance difference stands out in an area that should have even luminance when an image of a blue sky, white wall or the like is taken, and thereby resulting in deterioration of the image quality.
In the camera disclosed in Patent Document 2, the uneven luminance is prevented by making most light advance straight through the beam splitter. In other words, the most straight-advancing light is used for image-taking, and the remaining light is used for focus detection. However, the light amount for focus detection becomes insufficient though the uneven luminance can be eliminated in some degree, and thereby resulting in an inadequacy of the focus detection accuracy for low luminance objects.